Re: [RFC v3 2/4] mm: move PG_slab flag to page_type

From: Hyeonggon Yoo
Date: Sun Jan 29 2023 - 23:35:17 EST

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On Thu, Jan 12, 2023 at 05:27:24PM +0100, Vlastimil Babka wrote:
> On 12/18/22 11:18, Hyeonggon Yoo wrote:
> > For now, only SLAB uses _mapcount field as a number of active objects in
> > a slab, and other slab allocators do not. As 16 bits are enough for that,
> > use remaining 16 bits of _mapcount as page_type even when SLAB is used.
> > And then move PG_slab flag to page_type.
> >
> > As suggested by Matthew, store number of active objects in negative
> > form and use helper when accessing or modifying it.
> >
> > page_type is always placed in upper half of _mapcount to avoid
> > confusing normal _mapcount as page_type. As underflow (actually I mean,
> > yeah, overflow) is not a concern anymore, use more lower bits.
> >
> > Add more folio helpers for PAGE_TYPE_OPS() not to break existing
> > slab implementations. To preserve current behavior apply page policy
> > in PAGE_TYPE_OPS() and use PF_NO_TAIL for slab pages and PF_ANY for others.
> >
> > Remove PG_slab check from PAGE_FLAGS_CHECK_AT_FREE. buddy will still
> > check if _mapcount is properly set at free.
> >
> > Note that with this change, page_mapped() and folio_mapped() always return
> > false for a slab page.
> >
> > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
>
> Seems like quite some changes to page_type to accomodate SLAB, which is
> hopefully going away soon(TM). Could we perhaps avoid that?

If it could be done with less changes, I'll try to avoid that.

>
> - Alternative 1: convert only SLUB for now, let SLAB keep using PG_slab
> until it's gone.
>
> - Alternative 2: SLAB's s_mem field seems wasteful to be a full blown
> pointer, it could share the word with "active" if it was an offset from
> page_addr(), see how it's normally set up:
>
> void *addr = slab_address(slab);
> slab->s_mem = addr + colour_off;
>
> so basically we would store the colour_off and perform this calculation
> on-demand.

Thank you for suggestions, I prefer the latter approach.
Will refresh with that.

By the way, I think printk part is already ready so I sent this first.
( https://lore.kernel.org/linux-mm/20230130042514.2418-1-42.hyeyoo@xxxxxxxxx )

> One caveat is KFENCE's kfence_guarded_alloc() which seems to be creating
> some fake slab? And setting s_mem to something that maybe isn't an offset up
> to 16 bits from slab_address(). That has to be investigated. Worst case,
> SLAB is going away soon(TM) so it's fine if KFENCE only works with SLUB?

Will investigate, but if SLAB is going away anyway I guess that would be
okay.

> > ---
> > fs/proc/page.c | 13 ++----
> > include/linux/mm_types.h | 11 +++--
> > include/linux/page-flags.h | 77 ++++++++++++++++++++++++----------
> > include/trace/events/mmflags.h | 1 -
> > kernel/crash_core.c | 3 +-
> > mm/slab.c | 44 ++++++++++++-------
> > mm/slab.h | 3 +-
> > 7 files changed, 98 insertions(+), 54 deletions(-)
> >
> > diff --git a/fs/proc/page.c b/fs/proc/page.c
> > index 6249c347809a..e7524f21cefe 100644
> > --- a/fs/proc/page.c
> > +++ b/fs/proc/page.c
> > @@ -67,7 +67,7 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
> > */
> > ppage = pfn_to_online_page(pfn);
> >
> > - if (!ppage || PageSlab(ppage) || page_has_type(ppage))
> > + if (!ppage || page_has_type(ppage))
> > pcount = 0;
> > else
> > pcount = page_mapcount(ppage);
> > @@ -124,11 +124,8 @@ u64 stable_page_flags(struct page *page)
> >
> > /*
> > * pseudo flags for the well known (anonymous) memory mapped pages
> > - *
> > - * Note that page->_mapcount is overloaded in SLOB/SLUB/SLQB, so the
> > - * simple test in page_mapped() is not enough.
> > */
> > - if (!PageSlab(page) && page_mapped(page))
> > + if (page_mapped(page))
> > u |= 1 << KPF_MMAP;
> > if (PageAnon(page))
> > u |= 1 << KPF_ANON;
> > @@ -178,16 +175,14 @@ u64 stable_page_flags(struct page *page)
> > u |= 1 << KPF_OFFLINE;
> > if (PageTable(page))
> > u |= 1 << KPF_PGTABLE;
> > + if (PageSlab(page))
> > + u |= 1 << KPF_SLAB;
> >
> > if (page_is_idle(page))
> > u |= 1 << KPF_IDLE;
> >
> > u |= kpf_copy_bit(k, KPF_LOCKED, PG_locked);
> >
> > - u |= kpf_copy_bit(k, KPF_SLAB, PG_slab);
> > - if (PageTail(page) && PageSlab(compound_head(page)))
> > - u |= 1 << KPF_SLAB;
> > -
> > u |= kpf_copy_bit(k, KPF_ERROR, PG_error);
> > u |= kpf_copy_bit(k, KPF_DIRTY, PG_dirty);
> > u |= kpf_copy_bit(k, KPF_UPTODATE, PG_uptodate);
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 3b8475007734..6b04ae65241d 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -207,10 +207,13 @@ struct page {
> > atomic_t _mapcount;
> >
> > /*
> > - * If the page is neither PageSlab nor mappable to userspace,
> > - * the value stored here may help determine what this page
> > - * is used for. See page-flags.h for a list of page types
> > - * which are currently stored here.
> > + * If the page is not mappable to userspace, the value
> > + * stored here may help determine what this page is used for.
> > + * See page-flags.h for a list of page types which are currently
> > + * stored here.
> > + *
> > + * Note that only upper half is used for page types and lower
> > + * half is reserved for SLAB.
> > */
> > unsigned int page_type;
> > };
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 69e93a0c1277..07063d60efe3 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -107,7 +107,6 @@ enum pageflags {
> > PG_workingset,
> > PG_waiters, /* Page has waiters, check its waitqueue. Must be bit #7 and in the same byte as "PG_locked" */
> > PG_error,
> > - PG_slab,
> > PG_owner_priv_1, /* Owner use. If pagecache, fs may use*/
> > PG_arch_1,
> > PG_reserved,
> > @@ -482,7 +481,6 @@ PAGEFLAG(Active, active, PF_HEAD) __CLEARPAGEFLAG(Active, active, PF_HEAD)
> > TESTCLEARFLAG(Active, active, PF_HEAD)
> > PAGEFLAG(Workingset, workingset, PF_HEAD)
> > TESTCLEARFLAG(Workingset, workingset, PF_HEAD)
> > -__PAGEFLAG(Slab, slab, PF_NO_TAIL)
> > __PAGEFLAG(SlobFree, slob_free, PF_NO_TAIL)
> > PAGEFLAG(Checked, checked, PF_NO_COMPOUND) /* Used by some filesystems */
> >
> > @@ -906,42 +904,72 @@ static inline bool is_page_hwpoison(struct page *page)
> > }
> >
> > /*
> > - * For pages that are never mapped to userspace (and aren't PageSlab),
> > - * page_type may be used. Because it is initialised to -1, we invert the
> > - * sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
> > - * __ClearPageFoo *sets* the bit used for PageFoo. We reserve a few high and
> > - * low bits so that an underflow or overflow of page_mapcount() won't be
> > - * mistaken for a page type value.
> > + * For pages that are never mapped to userspace, page_type may be used.
> > + * Because it is initialised to -1, we invert the sense of the bit,
> > + * so __SetPageFoo *clears* the bit used for PageFoo, and __ClearPageFoo
> > + * *sets* the bit used for PageFoo. We reserve a few high and low bits
> > + * so that an underflow or overflow of page_mapcount() won't be mistaken
> > + * for a page type value.
> > */
> >
> > #define PAGE_TYPE_BASE 0xf0000000
> > -/* Reserve 0x0000007f to catch underflows of page_mapcount */
> > -#define PAGE_MAPCOUNT_RESERVE -128
> > -#define PG_buddy 0x00000080
> > -#define PG_offline 0x00000100
> > -#define PG_table 0x00000200
> > -#define PG_guard 0x00000400
> > +#define PG_buddy 0x00010000
> > +#define PG_offline 0x00020000
> > +#define PG_table 0x00040000
> > +#define PG_guard 0x00080000
> > +#define PG_slab 0x00100000
> >
> > #define PageType(page, flag) \
> > ((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
> >
> > -static inline int page_has_type(struct page *page)
> > +#define PAGE_TYPE_MASK 0xffff0000
> > +
> > +static inline bool page_type_has_type(unsigned int page_type)
> > {
> > - return (int)page->page_type < PAGE_MAPCOUNT_RESERVE;
> > + return ((int)page_type < (int)PAGE_TYPE_MASK);
> > }
> >
> > -#define PAGE_TYPE_OPS(uname, lname) \
> > +static inline bool page_has_type(struct page *page)
> > +{
> > + return page_type_has_type(page->page_type);
> > +}
> > +
> > +
> > +#define PAGE_TYPE_OPS(uname, lname, policy) \
> > static __always_inline int Page##uname(struct page *page) \
> > { \
> > + page = policy(page, 0); \
> > + return PageType(page, PG_##lname); \
> > +} \
> > +static __always_inline int folio_test_##lname(struct folio *folio) \
> > +{ \
> > + struct page *page = &folio->page; \
> > + \
> > return PageType(page, PG_##lname); \
> > } \
> > static __always_inline void __SetPage##uname(struct page *page) \
> > { \
> > + page = policy(page, 1); \
> > + VM_BUG_ON_PAGE(!PageType(page, 0), page); \
> > + page->page_type &= ~PG_##lname; \
> > +} \
> > +static __always_inline void __folio_set_##lname(struct folio *folio) \
> > +{ \
> > + struct page *page = &folio->page; \
> > + \
> > VM_BUG_ON_PAGE(!PageType(page, 0), page); \
> > page->page_type &= ~PG_##lname; \
> > } \
> > static __always_inline void __ClearPage##uname(struct page *page) \
> > { \
> > + page = policy(page, 1); \
> > + VM_BUG_ON_PAGE(!Page##uname(page), page); \
> > + page->page_type |= PG_##lname; \
> > +} \
> > +static __always_inline void __folio_clear_##lname(struct folio *folio) \
> > +{ \
> > + struct page *page = &folio->page; \
> > + \
> > VM_BUG_ON_PAGE(!Page##uname(page), page); \
> > page->page_type |= PG_##lname; \
> > }
> > @@ -950,7 +978,7 @@ static __always_inline void __ClearPage##uname(struct page *page) \
> > * PageBuddy() indicates that the page is free and in the buddy system
> > * (see mm/page_alloc.c).
> > */
> > -PAGE_TYPE_OPS(Buddy, buddy)
> > +PAGE_TYPE_OPS(Buddy, buddy, PF_ANY)
> >
> > /*
> > * PageOffline() indicates that the page is logically offline although the
> > @@ -974,7 +1002,10 @@ PAGE_TYPE_OPS(Buddy, buddy)
> > * pages should check PageOffline() and synchronize with such drivers using
> > * page_offline_freeze()/page_offline_thaw().
> > */
> > -PAGE_TYPE_OPS(Offline, offline)
> > +PAGE_TYPE_OPS(Offline, offline, PF_ANY)
> > +
> > +/* PageSlab() indicates that the page is used by slab subsystem. */
> > +PAGE_TYPE_OPS(Slab, slab, PF_NO_TAIL)
> >
> > extern void page_offline_freeze(void);
> > extern void page_offline_thaw(void);
> > @@ -984,12 +1015,12 @@ extern void page_offline_end(void);
> > /*
> > * Marks pages in use as page tables.
> > */
> > -PAGE_TYPE_OPS(Table, table)
> > +PAGE_TYPE_OPS(Table, table, PF_ANY)
> >
> > /*
> > * Marks guardpages used with debug_pagealloc.
> > */
> > -PAGE_TYPE_OPS(Guard, guard)
> > +PAGE_TYPE_OPS(Guard, guard, PF_ANY)
> >
> > extern bool is_free_buddy_page(struct page *page);
> >
> > @@ -1037,8 +1068,8 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
> > (1UL << PG_lru | 1UL << PG_locked | \
> > 1UL << PG_private | 1UL << PG_private_2 | \
> > 1UL << PG_writeback | 1UL << PG_reserved | \
> > - 1UL << PG_slab | 1UL << PG_active | \
> > - 1UL << PG_unevictable | __PG_MLOCKED | LRU_GEN_MASK)
> > + 1UL << PG_active | 1UL << PG_unevictable | \
> > + __PG_MLOCKED | LRU_GEN_MASK)
> >
> > /*
> > * Flags checked when a page is prepped for return by the page allocator.
> > diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> > index 412b5a46374c..8301912f8c25 100644
> > --- a/include/trace/events/mmflags.h
> > +++ b/include/trace/events/mmflags.h
> > @@ -113,7 +113,6 @@
> > {1UL << PG_lru, "lru" }, \
> > {1UL << PG_active, "active" }, \
> > {1UL << PG_workingset, "workingset" }, \
> > - {1UL << PG_slab, "slab" }, \
> > {1UL << PG_owner_priv_1, "owner_priv_1" }, \
> > {1UL << PG_arch_1, "arch_1" }, \
> > {1UL << PG_reserved, "reserved" }, \
> > diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> > index 87ef6096823f..1ea8198c26e1 100644
> > --- a/kernel/crash_core.c
> > +++ b/kernel/crash_core.c
> > @@ -482,13 +482,14 @@ static int __init crash_save_vmcoreinfo_init(void)
> > VMCOREINFO_NUMBER(PG_private);
> > VMCOREINFO_NUMBER(PG_swapcache);
> > VMCOREINFO_NUMBER(PG_swapbacked);
> > - VMCOREINFO_NUMBER(PG_slab);
> > #ifdef CONFIG_MEMORY_FAILURE
> > VMCOREINFO_NUMBER(PG_hwpoison);
> > #endif
> > VMCOREINFO_NUMBER(PG_head_mask);
> > #define PAGE_BUDDY_MAPCOUNT_VALUE (~PG_buddy)
> > VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
> > +#define PAGE_SLAB_MAPCOUNT_VALUE (~PG_slab)
> > + VMCOREINFO_NUMBER(PAGE_SLAB_MAPCOUNT_VALUE);
> > #ifdef CONFIG_HUGETLB_PAGE
> > VMCOREINFO_NUMBER(HUGETLB_PAGE_DTOR);
> > #define PAGE_OFFLINE_MAPCOUNT_VALUE (~PG_offline)
> > diff --git a/mm/slab.c b/mm/slab.c
> > index 7a269db050ee..eee46f71c4b8 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -2269,6 +2269,21 @@ void __kmem_cache_release(struct kmem_cache *cachep)
> > }
> > }
> >
> > +static inline unsigned int slab_get_active(struct slab *slab)
> > +{
> > + return ~(slab->page_type | PG_slab);
> > +}
> > +
> > +static inline void slab_inc_active(struct slab *slab)
> > +{
> > + slab->page_type--;
> > +}
> > +
> > +static inline void slab_dec_active(struct slab *slab)
> > +{
> > + slab->page_type++;
> > +}
> > +
> > /*
> > * Get the memory for a slab management obj.
> > *
> > @@ -2291,7 +2306,6 @@ static void *alloc_slabmgmt(struct kmem_cache *cachep,
> > void *addr = slab_address(slab);
> >
> > slab->s_mem = addr + colour_off;
> > - slab->active = 0;
> >
> > if (OBJFREELIST_SLAB(cachep))
> > freelist = NULL;
> > @@ -2510,8 +2524,8 @@ static void *slab_get_obj(struct kmem_cache *cachep, struct slab *slab)
> > {
> > void *objp;
> >
> > - objp = index_to_obj(cachep, slab, get_free_obj(slab, slab->active));
> > - slab->active++;
> > + objp = index_to_obj(cachep, slab, get_free_obj(slab, slab_get_active(slab)));
> > + slab_inc_active(slab);
> >
> > return objp;
> > }
> > @@ -2524,7 +2538,7 @@ static void slab_put_obj(struct kmem_cache *cachep,
> > unsigned int i;
> >
> > /* Verify double free bug */
> > - for (i = slab->active; i < cachep->num; i++) {
> > + for (i = slab_get_active(slab); i < cachep->num; i++) {
> > if (get_free_obj(slab, i) == objnr) {
> > pr_err("slab: double free detected in cache '%s', objp %px\n",
> > cachep->name, objp);
> > @@ -2532,11 +2546,11 @@ static void slab_put_obj(struct kmem_cache *cachep,
> > }
> > }
> > #endif
> > - slab->active--;
> > + slab_dec_active(slab);
> > if (!slab->freelist)
> > slab->freelist = objp + obj_offset(cachep);
> >
> > - set_free_obj(slab, slab->active, objnr);
> > + set_free_obj(slab, slab_get_active(slab), objnr);
> > }
> >
> > /*
> > @@ -2635,14 +2649,14 @@ static void cache_grow_end(struct kmem_cache *cachep, struct slab *slab)
> >
> > raw_spin_lock(&n->list_lock);
> > n->total_slabs++;
> > - if (!slab->active) {
> > + if (!slab_get_active(slab)) {
> > list_add_tail(&slab->slab_list, &n->slabs_free);
> > n->free_slabs++;
> > } else
> > fixup_slab_list(cachep, n, slab, &list);
> >
> > STATS_INC_GROWN(cachep);
> > - n->free_objects += cachep->num - slab->active;
> > + n->free_objects += cachep->num - slab_get_active(slab);
> > raw_spin_unlock(&n->list_lock);
> >
> > fixup_objfreelist_debug(cachep, &list);
> > @@ -2744,7 +2758,7 @@ static inline void fixup_slab_list(struct kmem_cache *cachep,
> > {
> > /* move slabp to correct slabp list: */
> > list_del(&slab->slab_list);
> > - if (slab->active == cachep->num) {
> > + if (slab_get_active(slab) == cachep->num) {
> > list_add(&slab->slab_list, &n->slabs_full);
> > if (OBJFREELIST_SLAB(cachep)) {
> > #if DEBUG
> > @@ -2783,7 +2797,7 @@ static noinline struct slab *get_valid_first_slab(struct kmem_cache_node *n,
> >
> > /* Move pfmemalloc slab to the end of list to speed up next search */
> > list_del(&slab->slab_list);
> > - if (!slab->active) {
> > + if (!slab_get_active(slab)) {
> > list_add_tail(&slab->slab_list, &n->slabs_free);
> > n->free_slabs++;
> > } else
> > @@ -2865,9 +2879,9 @@ static __always_inline int alloc_block(struct kmem_cache *cachep,
> > * There must be at least one object available for
> > * allocation.
> > */
> > - BUG_ON(slab->active >= cachep->num);
> > + BUG_ON(slab_get_active(slab) >= cachep->num);
> >
> > - while (slab->active < cachep->num && batchcount--) {
> > + while (slab_get_active(slab) < cachep->num && batchcount--) {
> > STATS_INC_ALLOCED(cachep);
> > STATS_INC_ACTIVE(cachep);
> > STATS_SET_HIGH(cachep);
> > @@ -3162,7 +3176,7 @@ static void *____cache_alloc_node(struct kmem_cache *cachep, gfp_t flags,
> > STATS_INC_ACTIVE(cachep);
> > STATS_SET_HIGH(cachep);
> >
> > - BUG_ON(slab->active == cachep->num);
> > + BUG_ON(slab_get_active(slab) == cachep->num);
> >
> > obj = slab_get_obj(cachep, slab);
> > n->free_objects--;
> > @@ -3297,7 +3311,7 @@ static void free_block(struct kmem_cache *cachep, void **objpp,
> > STATS_DEC_ACTIVE(cachep);
> >
> > /* fixup slab chains */
> > - if (slab->active == 0) {
> > + if (slab_get_active(slab) == 0) {
> > list_add(&slab->slab_list, &n->slabs_free);
> > n->free_slabs++;
> > } else {
> > @@ -3352,7 +3366,7 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
> > struct slab *slab;
> >
> > list_for_each_entry(slab, &n->slabs_free, slab_list) {
> > - BUG_ON(slab->active);
> > + BUG_ON(slab_get_active(slab));
> >
> > i++;
> > }
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 7cc432969945..c6ffe6799436 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -20,7 +20,8 @@ struct slab {
> > };
> > struct rcu_head rcu_head;
> > };
> > - unsigned int active;
> > + /* lower half of page_type is used as active objects counter */
> > + unsigned int page_type;
> >
> > #elif defined(CONFIG_SLUB)
> >
>

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